1. Field of Invention
The present invention relates to a method of manufacturing an organic EL (electroluminescent) element, and an organic EL element manufactured by the method.
2. Description of Related Art
In recent years, development of an organic EL element (a luminous element having a structure including an anode and a cathode, and a light emitting layer formed of an organic matter sandwiched between the anode and the cathode) has increasingly proceeded to provide a self-luminescent type display as an alternative to a liquid crystal display. Materials for the light emitting layer of such an organic EL element include, for example, a low molecular weight organic material, such as quinolinolatoaluminum complex (Alq3), and a high molecular weight organic material, such as poly(para-phenylene-vinylene) (PPV).
A film of a light emitting layer formed of a low molecular weight organic material is provided by a vapor deposition process, as is described in, for example, xe2x80x9cAppl. Phys. Left. 51(12), Sep. 21, 1987 pp. 913xe2x80x9d. A film of a light emitting layer formed of a high molecular weight organic material is provided by a coating process, as is described in, for example, xe2x80x9cAppl. Phys. Left. 71(1), Jul. 7, 1997 pp. 34+xe2x80x9d.
In many cases, a hole transporting layer is provided between the light emitting layer and the anode of the organic EL element. A hole is injected from the anode to the hole transporting layer, and the hole transporting layer transports the hole to the light emitting layer. When the light emitting layer has a capability of transporting a hole, the hole transporting layer is not always provided. In some cases, a hole injecting layer is provided as a layer other than the hole transporting layer. When the light emitting layer is formed of a high molecular weight material, such as PPV, a conductive polymer, such as a polythiophene derivative or a polyaniline derivative, is used as the hole transporting layer in many cases. When the light emitting layer is formed of a low molecular weight material, such as Alq3, a phenylamine derivative or the like is used as the hole transporting layer in many cases.
For example, in an organic EL element for display, anodes are formed in individual pixels on a substrate, and a light emitting layer and a hole transporting layer must be placed on each anode. Accordingly, if the light emitting layer and the hole transporting layer can be placed by ink jet process, coating (application) and patterning can be performed concurrently to thereby yield a patterning with a high precision for a short time. Additionally, this process is also effective to cut wastes in material and to reduce manufacturing cost, since a minimum of material is all that needs to be used.
To place the light emitting layer and hole transporting layer by ink jet process, liquid materials must be used. When a high molecular weight material such as PPV is used as the material for light emitting layer, the use of, for example, a solution of its precursor can place the light emitting layer by ink jet process. The placement of a light emitting layer that is formed of PPV high molecular weight material is described in, for example, Japanese Unexamined Patent Application Publication No. 11-40358, Japanese Unexamined Patent Application Publication No. 11-54270, and Japanese Unexamined Patent Application Publication No. 11-339957.
In the ink jet process, a region on which the light emitting layer and hole transporting layer are formed is surrounded with a barrier, and a liquid material is discharged toward the region surrounded with the barrier to thereby place the liquid material in the region. As the barrier, a two-layer structure barrier including a tower layer (substrate side) formed of an inorganic insulator, such as silicon oxide, and an upper layer formed of an organic polymer, such as polyimide is described in, for example, PCT International Publication No. WO99/48229.
FIG. 21 is a sectional view showing the aforementioned structure. Each of anodes 2 is formed in each pixel position on substrate 1, and lower barrier 31 formed of silicon oxide is formed so as to surround the periphery of each anode 2. Additionally, upper barrier 32 formed of polyimide is formed on lower barrier 31. Lower barrier 31 and upper barrier 32 are formed, for example, to a thickness (total thickness) of 1 to 3 xcexcm by thin-film formation and patterning operations.
In this connection, PCT International Publication No. WO99/48229 also describes the liquid-repellent treatment of the surface of the upper layer of the barrier by plasma treatment.
However, in the barrier of two-layer structure in which the upper barrier is formed of polyimide, the thickness of the light emitting layer might become uneven in the vicinity of the barrier and at the center, with regard to the height of the barrier, and of the affinity for polyimide of a liquid (a liquid containing a material for the formation of light emitting layer) discharged by ink jet process. When the thickness of the light emitting layer becomes uneven, the light emission color or the amount of light emission becomes uneven or unstable in the pixel to thereby deteriorate luminous efficacy.
In a color display in which red, green and blue three color pixels are placed adjacent to one another, it is necessary to ensure that different liquids are placed separately in adjacent pixels and to avoid the liquid in each pixel from being contaminated by another liquid for the adjacent pixel. The barrier having the aforementioned structure is also susceptible to being enhanced in this regard. In a contaminated pixel, purity of emitted light color is deteriorated.
In this connection, these problems can be enhanced also by the method described in PCT International Publication No. WO99/48229, but this method requires plasma treatment and is also susceptible to enhancement in, for example, cost.
The present invention has been accomplished focusing attention on these problems of the conventional technologies, and an object of the present invention is to ensure that a liquid is placed in a predetermined region (without placing the same in the adjacent region) to a uniform thickness in the region, when a light emitting layer or hole transporting layer constituting an organic EL element is placed by a liquid placing step such as ink jet process.
In order to solve the above problems, the present invention provides a method of manufacturing an organic EL element including a cathode, an anode, and one or more constitutive layers sandwiched between the cathode and anode, which constitutive layer includes at least a light emitting layer. The method includes the step of selectively placing a liquid containing a material for the formation of constitutive layer in a region for the formation of constitutive layer, using a pattern having an opening corresponding to the region for the formation of constitutive layer, in at least one constitutive layer, and, in the liquid placing step, an ultrathin organic film pattern having a surface being repellent to the liquid is formed as the pattern, using a compound having a functional group being bondable to the constitutive atom of a face, on which the film is formed, and a functional group being repellent to the liquid.
The liquid-repellency of the surface of the ultrathin organic film pattern preferably has such liquid-repellency that the contact angle of the liquid is 50xc2x0 or more.
In the method according to the present invention, it is preferred that at least one constitutive layer is further subjected to the step of forming an ultrathin organic film on a face on which the constitutive layer is formed, which ultrathin organic film has a surface having affinity for the liquid, using a compound having a functional group being bondable to the constitutive atom of a face on which the film is formed, and a functional group having affinity for the liquid, and this step is between the ultrathin organic film pattern forming step and the liquid placing step.
The term xe2x80x9cultrathin filmxe2x80x9d as used in the present invention means a thin film having a thickness of about several nanometers (e.g., 3 nm or less). Such an ultrathin organic films includes, for example, a self-assembled film. The aforementioned ultrathin organic film pattern is preferably a pattern formed of a self-assembled film.
The term xe2x80x9cself-assembled filmxe2x80x9d as used in the present invention means a monomolecular film which is obtained by allowing a compound having a straight-chain molecule and a functional group that can be combined with the constitutive atom of a face on which the film is formed and is bonded to the straight-chain molecule to become in the state of a gas or liquid and coexistent with the face on which the film is formed. In the monomolecular film, the functional group is adsorbed by the film-forming face and is bonded with the constitutive atom of the film-forming face, and the straight-chain molecule is formed facing the outside. This monomolecular film is referred to as a self-assembled film, since it is formed by spontaneous chemical adsorption of the compound to the film-forming face.
In this connection, the self-assembled film is described in detail by A. Ulman in Chapter 3 of xe2x80x9cAn Introduction to Ultrathin Organic Film From Langmuir-Blodgett to Self-assemblyxe2x80x9d (Academic Press Inc., Boston, 1991).
The liquid-repellent ultrathin organic film pattern includes a pattern formed of a self-assembled film which is formed by using a material having a fluoroalkyl group, such as a fluoroalkylsilane. In this case, the face on which the film is formed must be converted into hydrophilic.
When a self-assembled film is formed on a hydrophilic film-forming face (a film-forming face on which a hydrophilic group such as hydroxyl group exists) using a fluoroalkylsilane, a siloxane bond is formed by dehydration reaction with the hydroxyl group of the film-forming face, and a fluoroalkyl group (CF3(CF2)n(CH2)nxe2x80x94) is placed at the end of a straight-chain molecule, and the surface of the resulting self-assembled film becomes repellent to liquid (a property resistant to wetting with a liquid).
The ultrathin organic film forming step can be performed by the step of entirely forming an ultrathin organic film having a liquid-repellent surface using the compound, and the step of irradiating the ultrathin organic film with ultraviolet radiation via a photomask to thereby remove a portion corresponding to the constitutive layer forming region of the ultrathin organic film.
The lyophilic ultrathin organic film is preferably a self-assembled film formed of a material having an amino group or carboxyl group as a lyophilic functional group. When an amino group or carboxyl group is present on the surface, the film has a high affinity for, for example, water and alcohol which are generally used as a solvent in a material for the formation of hole transporting layer.
As the material for the formation of hole transporting layer, for example, a mixture of polyethylene dioxythiophene with poly(styrenesulfonic acid), or copper phthalocyanine is used. Accordingly, a self-assembled film formed by the use of an alkylsilane having an amino group or carboxyl group as a lyophilic functional group is preferably used as the lyophilic ultrathin organic film which is formed as an underlayer of the hole transporting layer. By this configuration, the adhesion of the material for the formation of hole transporting layer is enhanced, since the resulting self-assembled film has an amino group or carboxyl group on its surface.
As the material for the formation of light emitting layer, for example, polyfluorene-based polymer or poly(phenylene-vinylene)-based polymer is used. Accordingly, when the lyophilic ultrathin organic film is formed as an underlayer of the light emitting layer, a self-assembled film, which is formed by the use of an alkylsilane having, for example, an allyl group, vinyl group, phenyl group, or benzyl group as the lyophilic functional group, is preferably used as the lyophilic ultrathin organic film. By this configuration, the adhesion of the light emitting layer formed of a polyfluorene-based polymer or poly(phenylene-vinylene)-based polymer is enhanced, since the resulting self-assembled film has an allyl group, vinyl group, phenyl group, or benzyl group on its surface.
The present invention provides a method in which the liquid placing step in the invented method is performed by ink jet process. Specifically, when the organic EL element is manufactured as an element constituting a pixel of a color display in which red, green and blue three color pixels are placed adjacent to one another, the invented method is preferably employed when a liquid containing a material for the formation of light emitting layer and/or hole transporting layer is placed by ink jet process.
The present invention provides, in another aspect, an organic EL element which includes a cathode and an anode, and a light emitting layer, a hole injecting layer and/or a hole transporting layer sandwiched between the cathode and the anode, at least one of the light emitting layer and the hole injecting layer and/or the hole transporting layer is surrounded with a barrier, which barrier has a two-layer structure formed of a thin insulating film layer and an ultrathin organic film layer formed on the thin insulating film layer. The ultrathin organic film layer has a liquid-repellent surface and is provided by using a compound having a functional group being bondable to the constitutive atom of a face on which the film is formed, and a functional group being repellent to the liquid.
The thin insulating film constituting the barrier of this two-layer thin film preferably has a film thickness of 50 to 200 nm, and the ultrathin organic film layer preferably has a film thickness of 3 nm or less.